1. Field of the Invention
This invention relates generally to a wave soldering operation and, more particularly, to apparatus and associated methodology augmenting the soldering operation for mitigating warpage of printed wiring boards.
2. Description of the Prior Art
The introduction of semiconductor technology fostered a need for production line techniques which effected rapid soldering of numerous active and passive components to conductor paths affixed to an insulative panel, that is, to a so-called printed wiring board (PWB). The technique now known as wave soldering has proven to be successful in that the soldering operation rapidly produces solder connections which are electrically reliable as well as mechanically strong and resistant to fatigue.
The conventional wave soldering operation is characterized by moving the PWB, with the components mounted on the top surface and having their electrical leads projecting through to the underside of the PWB, laterally across the crest of a flowing stream of molten solder. As the board exits the stream, a quantity of solder clings to both the electrical leads and the conductor paths and eventually hardens to form a bond between the leads and paths.
A good solder bond is characterized by a fine grain structure and a well filleted layer of solder having a bright finish and making a relatively small angle of contact with the conductor paths on the PWB. A weak solder joint, conversely, is characterized by a coarse grain structure having crystal-like formations. In U.S. Pat. No. 3,605,244, issued to Osborne et al on Sept. 20, 1971, an arrangement is disclosed whereby heat is quickly extracted from the joint, thus preventing formation of excessively crystalline or grainy solder joints. With this arrangement, a cooling medium such as cold pressurized air or other gas is directed against unsoldered portions of the component leads exposed on the top side of the PWB. The board serves as an insulating shield between the cooling medium and solder joint so the solder is not dislodged by the pressurized gas while the component leads serve as heat conductors which dissipate heat from inside the joints to the cooling medium. The joint is thereby chilled in such a manner so as to minimize temperature differentials within the solder as well as accelerating the hardening process.
The Osborne et al disclosure is primarily directed towards apparatus which produces quality solder joints. One difficulty with the wave soldering operation not addressed in this or other prior art is that of producing quality bonds which simultaneously minimizing PWB warpage. During a wave soldering operation, heat is applied to a relatively narrow band of the PWB as it passes over the molten solder. A temperature differential exists in the transverse direction across the PWB as it exits the wave. The temperature characteristic across the narrow band of the board resembles a normal distribution with the centerline region being considerably hotter than the edges. If the temperature across the PWB can be held more constant, warpage may be reduced significantly. This reduction is critical since modern systems require numerous circuit packs or printed wiring boards to be mounted in close proximity in order to achieve high circuit density. Based on conventional wave soldering technology, the warpage requirements established for high density packing are virtually impossible to achieve.